Food-poisoning, ciguatera caused by poisoning of originally non-toxic fishes, widely occurs in coral reef islands region of subtropical and tropical regions, and more than 50,000 people suffer annually from ciguatera. Although the mortality is not so high, symptoms such as abnormal sensation, diarrhea, lassitude, arthralgia or itching last for several months under some circumstances. Ciguatoxins (CTX), which are isolated and the structure of which is decided as a main originated poison of ciguatera, are macromolecules characterized by fused 13 ether rings and their molecular length is approximately 3 nm, further more than 20 kinds of homolog are existing. Ciguatoxins are produced from dinoflagellate Gambierdiscus toxicus and accumulate in fishes by means of food chain. Since approximately 400 kinds of toxic fishes are normal from the view points of appearance, taste and odor, it is not safe to exploit fish sources of southern sea region. Therefore, the development of detective method of ciguatoxins by means of easy and high sensitive immunological measuring method of ciguatoxins is strongly expected.
Ciguatoxins bind specifically to voltage-sensitive Na+ channels (VSSC) of excitable membranes, activate it and generate toxicity, however, the activation mechanism of ciguatoxins at structural level is not made clear yet. Ciguatoxins exist in nature is very small and cultural production by the dinoflagellate is very slow, detail biological research and the preparation of anti-CTX antibody using natural product is virtually impossible. Under said circumstances, the quantitative supply of natural ciguatoxins by practical chemical synthesis is strongly desired.
Inventors of the present invention already proposed a total synthesis of CTX3C, which is one of main homolog of ciguatoxin (non patent document 1, Proc. Natl. Acad. Sci. U.S.A. 101, 1203-12018 (2004)). Further, the inventors developed a Sandwich immunoassay that can detect CTX3C easily (non patent document 2, J. Am. Chem. Soc. 125, 7608-7612 (2003)) and are now investigating to apply it to identification of a ciguatera fish. However, since CTX3C is mainly contained in a herbivorous fish, preparation of an antibody originated to other homolog is necessary for detection of ciguatoxin from a carnivorous fish.
CTX1B is the most typical ciguatoxin contained mainly in a carnivorous fish and has more complicated structure than CTX3C, and is known as the most historically important ciguatoxin whose structure is firstly decided in 1989. At the decision of structure, 0.3 mg of CTX3B isolated from 4000 kg of poisonous moray is used. However, since it was actually impossible to obtain practical amount of sample from nature, development of total synthesis of CTX1B is awaited for the actual use of CTX1B as a standard sample.
Generally, in total synthesis, if partial structure is different, development of a new synthesis route becomes necessary. However, for the purpose to synthesis many ciguatoxin homologs existing in nature in a unified fashion, the inventors have developed convergent total synthesis, which is characterized to be remarkably simple and more reliable compared with competitive methods. By said method, supply of over than several mg of CTX3C became possible up to this time. According to said concept, since there is possibility that carnivorous fishes accumulate ciguatoxin by higher concentration than herbivorous fishes because carnivorous fishes are locating at upper position of food chain than herbivorous fishes and is more dangerous as a ciguatera poisoned fish, the inventors of the present invention considered to develop a new effective total synthesis of CTX1B for the purpose of investigation of CTX1B.
At the development of a new effective total synthesis, the inventors of the present invention considered to utilize the reaction sequence which were already developed for synthesis of CTX3C. Namely, the inventors considered to apply the coupling of ABCDE ring segments with HIJKLM ring segments and subsequent construction of FG ring to CTX1B. However, since 7-members ring E-structure, and a side chain existing in A ring segments of CTX1B are structurally different from CTX3C, direct application of methodology used in CTX3C was impossible. Therefore, the inventors planned to develop a higher yielding process from a view point of effective preparation of the aimed compound.
(1) At the formation of 7-members ring of compound D at radical ring forming reaction, preparation process of CTX3C can not be used. Therefore, the inventors designed compound 5 that has pentafluoroacrylate instead of conventionally used methylacrylate and yield of ring forming reaction is remarkably improved.
(2) At deprotection of naphthylmethyl (NAP) group, side chain of A ring segment is very unstable to acid and compound E acetal intermediate is formed at conventional acid hydrolysis of acetal. Therefore, various investigations for condition are carried out and it is understood that acetal can be removed by condition of 1N hydrochloric acid/methanol, and the total synthesis of CTX1B can be carried out for the first time. Further, at above mentioned development, the inventors considered to utilize HIJKLM ring segments compound A, which was already reported in a paper (non-patent document 3, J. Org. Chem. 69, 2797-2804 (2004)), and compound C (non-patent document 4, J. Org. Lett., 6, 751-754 (2004)) as one of intermediates, according to the thinking that O,S-acetal compound 3, which is the most important intermediate, can be synthesized by coupling reaction developed by the inventors that permits neutral condition.